More than anything else, upright posture defines us as humans. It also defines the science of physical anthropology, which is the study of the evolution and design of the human body from head to foot. Although upright posture is one of the most positive aspects of evolution in humans it may play a role in many diseases of the brain and spinal cord and diseases of other organs of the body.
Upright posture has a particularly interesting impact on the design of the skull, spine and pelvis. It also has an impact on all the organs and circulatory systems contained within the compartments of those structures, such as the brain inside the head, the heart and lungs inside the ribcage and the abdominal organs, especially those contained within the relatively constricted confines of the pelvis, which bears the weight of all the organs above due to gravity. Consequently, upright posture plays a role in diseases of the internal organs, especially those located in the pelvis.
The Bent Base of the Skull
Starting at the head, physical anthropology shows that one of the key features that distinguish human skulls from four legged animals is the bend in the base of the skull. The bent skull base puts the large opening, called the foramen magnum for the passage of the brainstem into the spinal canal, beneath the center mass of the skull and brain. See Racial Skull Design – Square Heads vs Round Heads for further information on skull design.
This puts the brainstem in a rather precarious position perched directly over this large hole. It is kept in this position by flotation inside a water jacket filled with cerebrospinal fluid (CSF). It is CSF that keeps the brainstem from sinking into the foramen magnum, which would result in compression of the brainstem.
Humans also developed extra venous outlets and openings in the base of the skull to increase the drainage capacity of the brain. Some of those extra outlets are nearby or pass through the foramen magnum. The extra outlets are important in physical anthropology because they distinguish humans and hominids from other somewhat upright species such as apes.
The increase in drainage capacity of the brain was necessary to offset the increase in blood flow caused by upright posture. It also occured naturally as a response to upright posture. Over time the force of the downhill stream from upright posture slowly carved out the extra openings in the base of the skull. Only the best designs fit for survival in upright posture survived the change.
Compression of the brainstem and subsequent blockage of venous outlets as a result of the brainstem sinking into the foramen magnum can have profound and catastrophic affects on human health. For example, the design of the human skull may play an important role in neurodegenerative diseases such as Alzheimer’s, Parkinson’s and multiple sclerosis.
The S-Like Curves in the Spine
In light of comparative anatomy and physical anthropology, the skull isn’t the only structure that’s bent. The spine has four curves in it as well. The curves are not seen in four legged animals or other primates. They are truly human. Two of the curves, one in the neck and one in the low back, arch gently forward toward the chin and belly. The other two curves located in the ribcage and pelvic areas curve gently backwards toward the back of the body.
The curves in the spine make it easier to position the head directly over the perpendicular postural line. They also help to more evenly distribute the loads from front to back and give the spine a spring-like action. More importantly, the curves increase the strength and mechanical advantages of the spine. Lastly, the curves of the spine dissipate the compressive loads caused by upright posture down through opposing arches unlike a straight rod. This spares cartilage and connective tissue from excess stress by redirecting the stress away from compression.
The curves of the spine play a significant role in the cause of many diseases and conditions such as thoracic outlet syndrome, scoliosis, degenerated and herniated cartilage called discs, and even in spina bifida and hydrocele problems found in the lower cord. The design of the spine may play a key role in demyelinating diseases of the brain such as multiple sclerosis, and the lower cord, such as amyotrophic lateral sclerosis and progressive lateral sclerosis.
The spine has a huge impact on human health and disease. It is far too complex to go into here. For further information, please see the separate sections on the spine or the links below.
The Backward Bend in the Pelvis
Physical anthropology shows us that, like the skull and spine, the human pelvis is similarly bent. The pelvis starts in the base of the spine called the sacrum. Looking at it from the side, the sacrum arches slighly backwards toward the buttocks in between the two pelvic bones. The buttocks, hips and leg muscles attach to the pelvic bones.
The walls of the pelvis are formed by the innominate bones, which are made of three bones that join together called the ilia, ischia and pubic bones. The ilia form the upper part of the pelvis and the ischia form the bottom parts also known as the sitting bones. Rather than flat, the sitting bones have a slight rocker shape to their bottoms.
Most primates have ischial pads, that look like leather seats, that cover their sitting bones. Their ischial pads can be seen on the bottom of their buttocks near the crease where the buttocks muscles join the legs. The two pubic bones form an arch in the front of the pelvis and unite the three pelvic bones in the middle. The excretory and reproductive organs and their openings are all located in the compartment of the pelvis, which consists of the pubic bone in front, the sacrum and tailbone in the rear and the walls of the upper pelvis and sitting bones on the sides.
Looking at the pelvis from the side, the top and bottom parts bend backwards towards each other. The bend is in the area just above the sitting bones and is called the sciatic notch. The sciatic notch gets humans into a lot of trouble. For example, it is connective tissue and bone that form the notch that contains and can compress the sciatic nerve.
The design of the pelvis combined with the affects of gravity cause many diseases and health problems for humans such as menstrual issues, constipation, headaches and problems with pregnancy to name just a few. It can lead to tunnel entrapments of the sciatic nerve, as well as tethering of the tail end of the spinal cord (filum terminale) that attaches to the tailbone (coccyx). As an interesting aside, humans have more tail segments than some other primates. It is my opinion that humans use their tail segments to add structural strength to the floor of the pelvis.
Physical anthropology studies the difference in humans pelvic anatomy compared to other primates. It is surprising how close some are but among many of our differences, it is the bend in the pelvis that truly sets humans apart. The outward displacement of the hip sockets is another. While it is marvelous in design considering its intended use, as was said before it prediposes humans to disease of the organs housed inside its walls.
But physical anthropology is not just about disease. Physical anthropology is also about health. By studying the structure of the axial skeleton we can learn how to maintain its health and all the organs and structures that are contained within or attach to it.
The unique design of the human skull due to upright posture is further discussed on skull anatomy, skull shape, skull base, Skull Deformation and Correction and Brain Cooling and the Cranial Veins.